1. Field of Invention
The present invention relates to an image forming apparatus for effectively positioning an image forming member and a method of positioning the image forming member used in the image forming apparatus.
2. Description of the Background Art
Image forming apparatuses, such as copiers, printers, facsimile machines, and so forth, form an electrostatic latent image on an image bearing member. The electrostatic latent image is developed by a developing unit to a toner image to be transferred onto a recording medium. After the toner image is fixed by a fixing unit, the fixed image is finally output to a sheet discharging tray.
Some background image forming apparatuses use one image bearing member for producing a single color image, and some use a plurality of image bearing members for producing a multiple color image including a full-color image.
The above-described background image forming apparatuses for full-color images employ methods as described below. One of the methods is that a plurality of image bearing members forms respective single color toner image, and sequentially overlays the respective single color toner images of different colors onto a recording medium conveyed by a sheet feeding unit and/or a sheet transfer member. Another method is that a plurality of image bearing members forms respective single color toner images that are sequentially transferred onto a surface of an intermediate transfer member so that an overlaid color toner image is formed on the surface of the intermediate transfer member to further be transferred onto a recording medium. Further, another method is that one image bearing member forms respective single color toner images by turns and sequentially transfers these single color toner images directly onto a recording medium conveyed by an intermediate sheet transfer member.
A structure of an image forming apparatus having a plurality of image bearing members arranged in a line parallel to a sheet feeding direction of a belt-shape intermediate transfer member is well known as a tandem type structure.
An image forming apparatus employing the tandem type structure is generally required to have a plurality of image forming units, and therefore, the image forming apparatus is likely to become large in size and complicated in system structure.
To eliminate the problem, some techniques have been proposed. One of the techniques has proposed to have an image forming apparatus including a first image bearing member surrounded by two developing units having different colors of developers from each other and a second image bearing member surrounded by two other developing units having different colors of developers from each other and from the developing units disposed around the first image bearing member. Respective color toner images formed on the first and second image bearing members are transferred on an intermediate transfer belt.
With the above-described structure, the number of image bearing members disposed in an image forming apparatus can be reduced, for example from four to two, and therefore, the size in the image forming apparatus and the complexity in the system structure can be reduced or eliminated.
The above-described structure has two housing units, each of which can include respective image forming units. More specifically, one of the two housing units includes the first image bearing member and the two developing units disposed around the first image bearing member, and the other of the two housing unit includes the second image bearing member and the two developing units disposed around the second image bearing member. These housing units are arranged to have a minimum amount of space between them, and each of the housing units can detachably be attached to the image forming apparatus in a same direction to which a surface of an intermediate transfer belt is extended.
In the above-described structure, the developing units are fixedly disposed in the respective housing units. Therefore, when the housing units are attached to and detached from the image forming apparatus, an operator needs to separate the intermediate transfer belt from the housing units so as to obtain sufficient space for replacing the housing units without damaging the intermediate transfer belt. Therefore, a sufficient amount of space for replacing the housing units is required, and especially a space for separating the intermediate transfer belt from the housing units is required. For the above-described reasons, the size of the image forming apparatus may need to be increased. More specifically, the above-described technique involves a structure in which the housing units are moved in the same direction as extends the surface of the intermediate transfer member, making it necessary to have a sufficient space to avoid the housing units from contacting or damaging the surface of the intermediate transfer member when the housing units are replaced. However, the greater the space becomes, the larger the size of the image forming apparatus increases.
Further, the image bearing member and the developing units are precisely positioned relative to each other in the housing unit. However, when errors occur in a processing step and/or a positioning step, it may be difficult to obtain such precise positioning.